water Article Hydrochemical Characteristics and Quality Assessment of Shallow Groundwater in the Xinzhou Basin, Shanxi, North China Guanyin Shuai * , Jingli Shao *, Yali Cui, Qiulan Zhang and Yuntong Guo School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, China; [email protected] (Y.C.); [email protected] (Q.Z.); [email protected] (Y.G.) * Correspondence: [email protected] (G.S.); [email protected] (J.S.) Abstract: The hydrochemistry and quality of local shallow groundwater was assessed within the Xinzhou basin in Shanxi, North China. Piper diagrams, correlation analysis, principal component analysis, chloro-alkaline indices, ion proportion diagrams, and Gibbs diagrams were used to re- veal the hydrochemical characteristics and evolution mechanisms of groundwater. Besides, two indicators of sodium adsorption ratio and soluble sodium percentage, USSL and Wilcox diagrams, and water quality index models were used to evaluate the groundwater quality for irrigation and drinking. In general, groundwater in most areas of the basin is fresh water with total dissolved solid lower than 1000 mg/L. But there are salt water in some parts of the southern basin, with total dissolved solid higher than 1000 mg/L, due to industrial and domestic pollution. The hydrochemical facies of groundwater in most areas are HCO3-Ca and mixed HCO3-Ca·Mg·Na types, while it is HCO ·SO ·Cl-Na type in a small part of the basin. Nitrate pollution widely occurred in the basin 3 4 because of the use of nitrogenous fertilizers. The dominant mechanism controlling the evolution of groundwater chemistry compositions was the weathering of rock minerals. It mainly reflected in Citation: Shuai, G.; Shao, J.; Cui, Y.; − Zhang, Q.; Guo, Y. Hydrochemical the dissolution of carbonate minerals. And the carbonate dissolution is the major origins of HCO3 , 2+ 2+ + Characteristics and Quality Ca , and Mg in groundwater. However, Na-Ca exchange is the important source of Na . Shallow Assessment of Shallow Groundwater groundwater was suitable for irrigation and drinking except for some southern parts of the basin. in the Xinzhou Basin, Shanxi, North These results will be helpful for the protection and efficient management of groundwater in the China. Water 2021, 13, 1993. https:// Xinzhou basin. doi.org/10.3390/w13141993 Keywords: hydrochemical characteristics; sources of main ions; groundwater evolution; nitrate Academic Editor: Per-Erik Mellander pollution; water quality assessment Received: 4 June 2021 Accepted: 15 July 2021 Published: 20 July 2021 1. Introduction More than one third of the area in China has an arid to semi-arid climate and most Publisher’s Note: MDPI stays neutral of this is located in the north of the country. Groundwater is the most important water with regard to jurisdictional claims in published maps and institutional affil- resources and widely used in those areas. Because it is more abundant than surface iations. water; meanwhile, groundwater is extracted easily and its quality is generally better than the quality of surface water [1–5]. However, groundwater was increasingly affected in recent decades [6]. Moreover, regional groundwater pollution has become a very common problem in arid-semiarid areas with the growth of population and the development of industry and agriculture [7–9]. Therefore, it is necessary to investigate and acquaint the Copyright: © 2021 by the authors. status, origin, and evolution of groundwater major ions in arid-semiarid areas, in order to Licensee MDPI, Basel, Switzerland. solve current water environment problems and achieve the sustainable utilization of water This article is an open access article distributed under the terms and resources. In addition, it is also important to evaluate groundwater quality for drinking conditions of the Creative Commons and irrigation. Attribution (CC BY) license (https:// Many studies on the chemical characteristics and quality assessments of groundwater creativecommons.org/licenses/by/ have been carried out [10–13]. Li Peiyue evaluated the status and evolution of groundwater 4.0/). Water 2021, 13, 1993. https://doi.org/10.3390/w13141993 https://www.mdpi.com/journal/water Water 2021, 13, 1993 2 of 16 compositions in two coalfields, and found that natural factors, like geological and hydro- geological conditions, as well as human activities had a combined effect on groundwater compositions, which can lead to the difference of groundwater chemistry characteristics in different areas [11,14]. Su Zhenmin investigated chemical characteristics of ground- water in Yinchuan area, and proved that surface water seepage, other aquifers leakage, and hydrogeological processes such as mineral dissolution and their speed of dissolution, as well as ion exchange played an important role in controlling the chemical evolution of groundwater [15–17]. However, when one or more kinds of groundwater chemistry compositions exceed their standard limits, it means groundwater pollution occurs. More- over, groundwater pollution has a great impact on life and production. Therefore, various methods have been developed to assess the potential risks of groundwater pollution to human health and ecological safety. Tian Hui employed the water quality index model (WQI) widely used over the world to assess the suitability of groundwater for drinking purposes in the Hailun area, and then got the spatial distribution of the water quality index based on the evaluated results [2]. Li Peiyue used three indicators, which were sodium adsorption ratio (SAR), soluble sodium percentage (%Na), and residual sodium carbonate (RSC), to assess irrigation water quality in a mining area, and found local groundwater was suitable for agricultural purposes [11]. The Xinzhou basin, located in a typical arid-semiarid region, is an important agricul- tural district where the area of cultivated land accounts for about 75%. It also distributes scattered coal mines and gold mines. The life of local residents, agricultural irrigation, and industrial production rely heavily on groundwater due to the shortage of surface water. Thus, it is vital to ensure the safety of local groundwater resources. Han Dongmei studied hydrochemical characteristics in some small areas of the Xinzhou basin, including the Yangwu river alluvial fan, the Daying and Qicun geothermal system. And her study revealed that groundwater chemistry was dominated by water-rock interaction; mixing between thermal and non-thermal groundwater can change the chemical compositions of groundwater [18–20]. Sun Jing and Zhang Xiaolun also discussed the hydrochemical characteristics of local geothermal fields [21,22]. While previous research has focused on the hydrochemical characteristics and evolution of small areas within the Xinzhou basin, this and groundwater quality status has to our knowledge not been studied within the whole basin. Therefore, this paper focuses on the groundwater of the whole basin and the following research is done: (1) describing the hydrochemical characteristics of shallow groundwater in the whole basin, including the concentrations of various ions, the status of ions exceeding their standard limits, and hydrochemical types; (2) analyzing the origin of main ions and the reason of nitrate pollution; (3) discussing the evolution mechanisms of groundwater chemistry in the basin; (4) evaluating the quality of local shallow groundwater; (5) proposing suggestions for pollution control. 2. Study Area The Xinzhou basin is located in the middle-northern part of Shanxi province between latitudes 38◦120–39◦270 N and longitudes 112◦130–113◦570 E, covering an area of 3385.2 km2 (Figure1). It is in the arid-semiarid area of north China with annual average evaporation of about 1600 mm and annual precipitation of 418 mm [19]. Hutuo River is the largest river in the basin and flows through the whole basin. Its main tributaries include Yunzhong River, Muma River, and Yangwu River, etc. Water 2021, 13, x FOR PEER REVIEW 3 of 17 And the boundary between these two plains is approximately the Yunzhong River. The former, a long strip shape area in a direction of NE40°–70°, is located in the north of the Yunzhong River, mainly composed of quaternary alluvial proluvial sediments. The latter is located in the southeast of the Yunzhong River, and mainly composed of quaternary alluvial lacustrine sediments (Figure 1). The strata outcropping in the study area include magmatic and metamorphic rocks of Archean age, Cambrian-ordovician dolomite and limestone, Carboniferous-Jurassic layered sandstone and shales, Tertiary basalt, and various Holocene and late Pleistocene sediments. These sediments contain alluvial and lacustrine sand, sandy loam, silt, and silty clay. The strata below the Cenozoic sediments in the basin are mostly the Archean igneous and metamorphic rocks, and locally Cambrian limestone. Cenozoic sediments range between 50 and 2100 m thickness including about 50 to 360 m of Quaternary sedi- ments [19]. The Quaternary sediments in the plain constitute the shallow aquifer of pore water. The thickness of this aquifer is over 130 m. Hutuo River is its surface watershed. The main recharge sources of this aquifer are lateral influx of groundwater from mountain areas, as well as precipitation and irrigation water infiltration. Controlled by topography, the shal- low groundwater flows mainly from the piedmont on both sides of the basin to the Hutuo Water 2021, 13, 1993 River valley and from the upper reaches of the Hutuo River to the